Allows ua.invite with 'media' key in options.

Up until recently we used 0.5.0 and

    var options = {
        media: mediaStream
      };
    ua.invite(target, options);

were "mediaStream" is an existing stream. (I believe that comes straight
from the [documentation](http://sipjs.com/guides/reuse-mediastreams/))

Today we switched to master and noticed that we got prompted with
Chrome's "wants you to use camera..." twice. It looks like the invite
now expects different options parameters and does not respond to
`mediaHint.stream`. Therefore we call getUserMedia again.

I wanted to keep it simple and change as little as possible.

While running/writing the tests I ran into some issues
(that I documented in BUILDING.md).